Thrombus formation in the arterial vasculature is a process initiated by the interaction of several platelet receptors with collagen and collagen-associated proteins at the site of vascular injury. Investigators have determined that, initially, platelets are tethered transiently to exposed collagen when the receptor GPIbα interacts with collagen-bound von Willebrand factor (vWF) (Ruggeri et al. 2002). For stable platelet adhesion to occur, the immunoglobulin (Ig)-like receptor GPVI must bind to collagen, triggering the activation of a signaling cascade (Moroi et al. Thromb Res. 2004), and Kahn et al. Semin Thromb Hemost. 2004). GPVI signaling leads to inside-out activation of the platelet integrins α2β1 and α11bβ3 (Nieswandt et al. EMBO J. 2001, and Chen et al. Mol Cell Biol. 2003). Activated α2β1 binds tightly to a specific sequence in collagen to allow firm adhesion of the platelets to the site of injury and activated α11bβ3 mediates platelet aggregation (Bennett et al. J Clin Invest. 2005). In addition, GPVI signaling stimulates secretion of platelet granule contents to activate nearby circulating platelets and propagate thrombus formation. In humans, GPVI deficiency causes a loss of platelet activation in response to collagen, and GPVI polymorphisms have been linked to increased risk of myocardial infarction. Remarkably, loss or inhibition of GPVI prevents arterial thrombus formation in animal models but causes only mildly prolonged bleeding times in mice and humans, suggesting that GPVI could be a prime therapeutic target for prevention of arterial thrombotic diseases such as heart attack and stroke.
The gene for GPVI is found in the leukocyte receptor cluster (LRC) on human chromosome 19. The sequence of the GPVI ectodomain was predicted to form two Ig-like domains comprising the collagen-binding domain followed by a heavily O-glycosylated stalk. Like other LRC receptors, GPVI associates with the FcR γ-chain co-receptor, and signaling is mediated both indirectly through the γ-chain and directly through the GPVI cytoplasmic domain. The quaternary structure of fibrous collagen is required for GPVI activation, although GPVI can also be activated by a synthetic collagen-related peptide (CRP) containing crosslinked strands of the repeating tripeptide (POG)n, where P is proline, O is hydroxyproline, and G is glycine (Farndale et al. J Thromb Haemost. 2004). Recently, GPVI has been shown to interact with the ectodomain of GPIbα on the surface of platelets (Arthur et al. Thromb Haemost. 2005) and to bind to laminin, a matrix protein exposed at sites of vascular injury (Inoue et al. Blood. 2005). Multimeric snake venom proteins such as convulxin can also strongly activate GPVI (Lu et al. Toxicon. 2005), suggesting that clustering of GPVI receptors through multiple binding events leads to activation. To better understand the molecular basis for collagen activation of platelets by GPVI, determining the crystal structure of the collagen-binding domain (CBD) of human GPVI and characterizing its interaction with CRP is highly desirable.